1. Field of the Invention
The present invention relates to a novel process for the production of sequences of interference layers. The invented process permits the production of interference layer systems of varying buildup, which can be used in a large variety of applications. These interference layers systems are suited, in particular, as filters, as interference pigments or as particles to be embedded in documents to prevent counterfeiting.
2. Description of the Related Art
Interference layer systems are composed of any desired number of layers having at least two different refractive indices and layer thicknesses, which are usually smaller than the respective light wavelengths. The interference layers are employed particularly as antireflex layers, reflection layers, interference pigments, beam splitters, edge filters, line filters and minus filters.
There are a number of processes known for producing interference layer systems. These processes are usually coating processes. Thus, chemical processes, such as for instance sol-gel coating processes, spraying processes, surface-reduction processes or CVD (chemical vapor deposition), as well as physical processes, such as for instance vaporization processes or sputtering, can be employed to produce interference layers. A large palette of high-refractive-index and low-refractive-index materials is available as coating and substrate materials.
Interference pigments for mother-of-pearl or metal glaze in lacquers, paints or cosmetics are predominantly produced by coating small platelike mica crystals with TiO2 or other metal oxides (cf. e.g. U.S. Pat. Nos. 3,553,001 or 3,331,699). Carbon-containing layers and basic organic dyes are also applied as coatings (cf. e.g. EP-0634458 or DE 4225357). Small mica plates with a diameter of 100 to 500 xcexcm and a thickness of 0.1 to 10 xcexcm painstakingly gained from natural mica, with a yield of less than 10%, are used as substrates. Other materials are also employed as substrates for interference coatings, such as for example PBSO4, small hexagonal Fe203 plates (diameter 5 to 50 xcexcm) and graphite. U.S. Pat. No. 5,436,077 also describes the use of small glass particles as a substrate on which a metal layer and a covering protective layer are deposited. However, mica-based mother-of-pearl and metal glaze pigments are most common and have more than 80% of the market.
The state of the art also describes-coating organic substrates with TiO2 or ZrO2 (cf. ZA-6 805 748) or coating preformed polyester-resin-based organic parts of molds (cf. EP-742 262) in order to produce interference pigments. However, the thermal and mechanic stability of these interference pigments generally do not meet the needed requirements.
A drawback of all the known processes and coating materials is the unsatisfactory precision in obtaining the refractive index and thickness of the layers so that the desired interference effect is often not achieved. Especially the refractive index is met only very inadequately due to the employed coating methods. Moreover, the production of sequences of interference layers is very complicated and requires many different process steps, each of which can harbor errors and increase the expense of the entire process. Furthermore, with these processes and materials, a large amount of the layer-forming materials is lost.
Moreover, the layers of the interference layer systems made with these conventional production processes cannot be obtained completely poreless so that the spectral properties may change as water vapor and other gases collect in the pores of the filter. Upon warming up, the gas charge of this interference layer system changes again so that the spectral properties, such as reflection and transmission, vary. In the most unfavorable conditions, e.g. due to environmental influences, the applied coating may also detach from the substrate.
U.S. Pat. No. 3,711,176 describes another process for the production of sequences of interference layers. In this process, high-reflective-index colored plastic films are made from multiple transparent, thermoplastic plastic layers by means of simultaneous extrusion. However, the resulting layer system is usually only suited for obtaining special optical effects on the surfaces due to the inadequate precision in attaining the refractive indices and the layer thicknesses. Moreover, the variation range of the refractive indices of the thermoplastics is only small, which greatly limits the ability to produce a selected interference effect. Moreover, the employed materials are not very thermally, mechanically, chemically or environmentally, e.g. UV radiation, resistant.
The object of the present invention is to provide a process for the production of a sequence of interference layers which permits producing precise and durable interference layer systems at low costs. Furthermore, to provide a sequence of interference layers producible thereby.
The object is solved by providing a process for the production of one of interference filters, interference pigments or interference particles having sequences of interference layers composed of layers i of prescribed thicknesses d(i) and refractive indices n(i), comprising the steps of providing a stack of glass plates having plane surfaces, the stack comprised of at least two layers i of glasses having refractive indices n(i) and thicknesses d0(i), which are each larger than the prescribed thicknesses d(i) by the same multiplying factor; heating the stack to a temperature above the transformation temperature of the glasses of the layers; drawing the stack during or after heating in such a manner that the respective layers obtain the prescribed thicknesses d(i); and cooling of the drawn stack.
The object is solved by means of the process according to claim 1 and the sequence of interference layers according to claim 18. Advantageous embodiments of the process are the subject matter of claims 2 to 16. An intermediate product, which is obtained in one embodiment of the process is set forth in claim 17.
The invented process permits precise and inexpensive production of sequences of interference layers of multiple layers i of prescribed thicknesses d(i) and refractive indices n(i), for example a layer sequence of layers 1 (i=1) and 2 (i=2) of varying thicknesses d(1), d(2) and varying refractive indices n(1) and n(2).
An element of the present invention is that it was understood that the production of such a type of sequence of interference layers cannot be realized using conventional coating methods or materials but rather in a very advantageous manner by using glass as the layer material in conjunction with the following process steps.
In this process, first a stack of at least two glass layers i composed of different glasses respectively types of glass, with the same refractive indices n(i) and the same layer sequence as in the to-be-produced sequence of interference layers, are provided. The thicknesses d0 (i) of the layers of the stack are selected in such a manner that they are always larger by the same factor than the prescribed thicknesses d(i) of the layers of the to-be-produced interference sequence. These parameters can be met in an excellent manner, in particular, with glass materials. The stack is finally heated to a temperature above the transformation temperature of the glasses of the layers and subsequently or simultaneously drawn in such a manner that the individual layers reach the prescribed thicknesses d(i). After this the drawn stack is cooled.
Strikingly, when drawn, the individual glass layers do not melt together in such a manner that they lose their original properties. But rather, both the refractive indices and the layer thicknesses ratio are exactly retained even in the case of large surface layers. The layers melt together only where they touch thereby creating a very advantageous strong bond between the individual precisely defined layers.
In a preferred embodiment of the invented process, the stack is provided as a so-called xe2x80x9cpreformxe2x80x9d. This preform comprises a stack of glass plates whose refractive indices n(i) and thicknesses d0 (i) are suitably selected according to the aforementioned conditions. The preform is drawn at temperatures above the transformation temperature in such a manner that the glass plates melt together where they touch and the thicknesses of the individual glass plates are reduced to the desired thickness. In simple words, one could say that the starting stack, in the invented process the preform, undergoes a similarity transformation, with the starting thicknesses d0 (i) and the starting widths b0 (i) being reduced by a constant factor while retaining the refractive indices. The area (parallel to the layering) yielded after drawing is enlarged by this factor. Setting the prescribed thicknesses d(i) preferably occurs via the drawing velocity. This can be exactly precalculated for each desired thickness reduction. The optical thicknesses n(i)xe2x80xa2d(i) of the individual glass plates i are usually selected in such a manner that, depending on the desired interference effect, xcex/4-layers or multiples thereof are created.
In another embodiment of the invented process, the drawing velocity is altered step by step or continuously during drawing. In this manner, successive interference layer sequences of varying thicknesses, i.e. with different spectral properties, are created from a single stack. In the case of continuously increase of the drawing velocity, a sequence of interference layers with continuously changing layer thicknesses along the drawing length (sequence of graded interference layers) can also be produced.
Alternatively, the provided stack for producing a graded interference filter can also have an already prescribed gradient thickness of the layers transverse to the drawing direction. Drawing then can occur at a constant rate.
In an advantageous embodiment, the preform subject to the drawing step is itself already composed of multiple single preforms each containing a stack of layers of a single sequence of interference layers respectively. The individual preforms are separated by intermediate layers of soluble thermoplastic materials, preferably glass. Soluble materials in this context refers to materials of the layers that are soluble, for instance, in an aqueous or an acidic solution which does not dissolve the glasses of the other layers. Examples of such material/solvent combinations are borosilicate glasses that are soluble in a weak acidic medium. After the drawn preform has cooled, the intermediate layers are dissolved in the suited solvent in such a manner that the individual sequences of interference layers are separated. The separation glasses of the intermediate layers can be applied by means of powder technology or screen-printing processes. This is also the case for the layers of the interference layer systems. For the preform, preferably either high-refractive-index or low-refractive-index glass is provided as thin glass and the respective other glass is applied onto it by means of screen printing or powder technology. The glass applied as frit has to be transformed into a transparent, homogeneous layer by means of thermal treatment. This can occur before assembling the individual layers to form a preform or at the same time as the melting together of the individual layers of the preform prior to the drawing procedure. Dependent on the flow behavior of the frit paste, it may be necessary to apply several layers per screen printing in order to obtain the required thickness.
This possible method of production of the preform is, of course, not limited to this preferred embodiment.
In the case of large starting layer thicknesses d0 (i), it may be necessary to carry out the drawing procedure in several steps. In this event, between the individual steps, differently or similarly layered packages of glass can be added to the layered package. The stack can, for instance, be separated into several longitudinal sections following one drawing step. These sections are placed one on top of the other to form a new stack which for its part undergoes the remaining drawing steps.
In another advantageous embodiment of the invented process, the stack is wound around a drum during the drawing step. In this way, several layers of the stack come to lie on top of each other. This multi-layer system is then divided into one or several sections and at least one of the sections, as a new stack, undergoes the remaining steps.
For better handling, the stack can be applied between two steps onto a carrier plate made of thermoplastic material, in particular glass or plastic, and undergoes the remaining steps with this carrier plate. The carrier plate can already be connected to the stack prior to starting the first drawing step. In the event that the carrier plate is not needed for stabilization of the produced sequence of interference layers after termination of the drawing step, the material of the carrier plate can be selected of a soluble material, such as was selected for the intermediate layers so that the carrier plate can be dissolved in a solvent.
Following cooling, the drawn stack can, if need be, be separated mechanically into individual particles or sections. For the production of interference pigments, this mechanical separation can, for example, occur using a cutting mill.
Possible glass materials are oxidic and non-oxidic glasses. The choice of types of glass should be such that, in addition to the refractive indices, compatibility of the glasses with regard to thermal drawing coefficients and interdiffusion as well as resistance to environmental influences are taken into account.
The entire drawing process occurs using a similar technique as employed in drawing optical light guide fibers. Surprisingly, such a process can also be applied to the present packages of glass plates in such a manner that after the drawing process, glass ribbons of defined glass layers are created. The individual glass layers can be produced with utmost precision with regard to thickness and refractive index in such a manner that the desired interference behavior can be obtained. The changes in reflection, transmission, absorption and polarization of an electromagnetic wave in the optical spectral range (UV, visible spectral range, IR) upon impinging upon the invented interference systems can be defined and set precisely. The interference systems produced in this manner are composed of firmly melted together layers of glass. There are no pores in the layers, and the layers adhere to each other extremely well. Consequently, the interference systems produced according to the present invention do not undergo any changes in their spectral properties after the production process. With the suited selection of glasses facing outward, resistance to most environmental influences of the interference systems is extremely good. They are transparent and UV-resistant.
Although the available range of high-refractive-index and low-refractive-index, oxidic and non-oxidic glasses is smaller than is the case with materials that are employed for the traditional coating processes. However, glasses with almost every refractive index between the extreme value nd=1.437 and nd=approx. 3.2 are known. For the invented purposes, glasses that are in normal use, i.e. with greater thicknesses, not transparent but transparent in the invented layer thicknesses can also be employed.
It is a particular advantage that the refractive indices of the used starting glasses can be met more precisely by several orders of magnitude compared to conventional coatings. The same holds for the layer thicknesses.
Furthermore, it is advantageous that the invented process permits producing interference systems with very many layers in a simple manner, because the most important step of the process, the drawing procedure, is for the most part independent of the number of glass plates of which the preform is composed.
Another, especially economical advantage is that the employed materials are practically completely utilized in making the interference layer system.
In some applications, an inherent advantage of the present invention can be exploited by making parts of the xe2x80x9cribbonxe2x80x9d being produced with varying thickness during the final drawing procedure. This can, as already explained, be realized by means of different drawing velocities. The interference systems being produced then all possess the same layer sequence, but different thicknesses, which leads to shifting the position of the spectral distribution of the interference system. Thus, for example, in the case of line filters, the spectral position of the greatest transmission range shifts.
Suited selection of the number of layer sequences, refractive indices and layer thicknesses permits obtaining any desired spectral distribution of reflection respectively transmission of the interference system.